WORK OF THE PRESENT - 113 



The larger problems of stratigraphy, correlation, oscillations between 

 land and sea, the migrations of faunas, lines of descent, parallel develop- 

 ment, etcetera, are all awaiting the student. The extent of the land 

 areas and the variations in character, thickness, and distribution of the 

 marginal and deep-sea deposits are imperfectly known. Structural and 

 dynamic problems of the most far-reaching importance are awaiting solu- 

 tion. If the principle is accepted that the classification and delimitation 

 of the divisions of the Paleozoic, Mesozoic, and Cenozoic eras must rest 

 on the broad biologic characters of their included faunas and floras, and 

 not on local breaks or differences of sedimentation, important problems 

 remain as to where these lines of demarkation shall be drawn in most 

 geologic provinces. 



, In studying the problems connected with the occurrence of ore de- 

 posits, the determination of their continuation in depth is of general in- 

 terest. The terrestrial chemistry of ore deposits is one of the most im- 

 portant problems and requires investigation in many regions. The 

 geologic conditions under which ores were originally deposited, whether 

 they are all forms of concentration by aqueous solution or in part con- 

 centration from eruptive magmas, are subjects for continued study. 



The field for research in physical geology is almost boundless, and 

 so little has it been cultivated that the harvest will surely be abundant. 

 Thus far geology has been pursued mainly by biologists, mineralogists, 

 and stratigraphers. A few physicists have, indeed, applied their pro- 

 fessional knowledge to the elucidation of the past and present condition 

 of the earth and have reached results of first importance. Physicists, 

 however, rarely have a sufficient acquaintance with geology to become 

 deeply interested in the subject, while geologists seldom have a firm 

 grasp of physics. A new school of geologists is needed, whose pre- 

 paratory training should be mathematical and experimental physics, as 

 that of a paleontologist is zoology. 



Physical geology begins with the primeval nebula and the genesis of 

 the earth-moon binary system. The causes of the heterogeneity of the 

 earth's density evinced by the distribution of oceans and land masses 

 and of the retardation of the earth's rotation and its effect on the arrange- 

 ment of continental outlines are yet unknown. The thermodynamic 

 problems of upheaval and subsidence, and the questions of deformation, 

 of compression, of plastic solids and of rupture as affecting mechanics 

 of orogeny, must be established. The effects of changes of climate in 

 geologic time, the causes of glacial epochs, volcanic phenomena, the 

 physics and chemistry of high temperatures, and many other problems 

 of geologic physics have }^et to be determined. The solution of these 

 problems must fundamentally affect the science of geology. 



